Stents are widely used for supporting a lumen structure in a patient's body. For example, a stent may be used to maintain patency of a coronary artery, other blood vessel or other body lumen such as the ureter, urethra, bronchus, esophagus, or other passage. A stent is typically a metal, tubular structure, although polymer stents are known. Stents can be permanent enduring implants, or can be bioabsorbable at least in part. Bioabsorbable stents can be polymeric, bio-polymeric, ceramic, bio-ceramic, or metallic, and may elute over time substances such as drugs.
In certain stent designs, the stent is an open-celled tube that is expanded by an inflatable balloon at the deployment site. Another type of stent is of a “self-expanding” type. A self-expanding stent does not use a balloon or other source of force to move from a collapsed state to an expanded state. A self-expanding stent is passed through the body lumen in a collapsed state. At the point of an obstruction, or other deployment site in the body lumen, the stent is expanded to its expanded diameter for its intended purpose. An example of a self-expanding stent is a coil structure that is secured to a stent delivery device under tension in a collapsed state. At the deployment site, the coil is released so that the coil can expand to its enlarged diameter. Coil stents can be manufactured using a variety of methods, such as winding of wire, ribbon, or sheet on a mandrel or by laser cutting from a tube, followed by the appropriate heat treatments. Another type of self expanding stent is an open-celled tube made from a self-expanding material, for example, the Protégé GPS stent from ev3, Inc. of Plymouth, Minn. Open cell tube stents are commonly made by laser cutting of tubes, or cutting patterns into sheets followed by or preceded by welding the sheet into a tube shape, and other methods.
The shape, length and other characteristics of a stent are typically chosen based on the location in which the stent will be deployed. However, selected segments of the human vasculature present specific challenges due to their shape and configuration. One such situation involves the ostium of short renal arteries within the human body.
Conventional stents are generally designed for segments of long cylindrical vessels. When such stents are deployed at the ostium of short renal arteries, in an attempt to prevent further progression of arteriosclerotic disease from the aorta into the renal arteries, they may extend into the aorta and disrupt the normally laminar blood flow. This result further compounds an existing need to minimize disruption of the flow pattern at the ostium. In addition, stents are hard to position on a consistent basis at the precise ostial location desired, and placement of renal stents can release arteriosclerotic debris from the treatment area. Such debris can flow distally into the kidney and embolize, causing impaired renal function.
Accordingly, it is desirable to flare the end of a stent to minimize disruption to flow patterns at an ostium and to not create barriers to future access by interventional devices. However, existing stents are hard to flare with existing expansion means. Stents suitable for expansion of renal arteries must have high radial strength when expanded to resist vessel forces tending to radially collapse the stent. This need for high stent strength makes suitable stents difficult to flare at the ostium. A stent configuration designed to address these concerns is disclosed in commonly assigned U.S. patent application Ser. No. 10/816, 784, filed Apr. 2, 2004, by Paul J. Thompson and Roy K. Greenberg, US Publication Number U.S. 2004/0254627 A1. However, this stent, when flared, has a lower percentage of stent strut coverage of vessel wall at flared regions than is desirable. It is known that stent struts, when expanded into contact with the vessel wall, should cover a certain percentage of the internal vessel wall area in order to prevent prolapse of tissue through the open spaces between stent struts.
Accordingly, a need exists for a stent that can be placed at the renal ostium which is both strong and provides a high percentage of vessel wall coverage.
Further need exists for a stent that will minimize disruption of the flow pattern at the ostium and which will lower the risk of embolization during deployment.